Transcriptional regulation is one of the major mechanisms that allow expression of tissue specific genes in distinct temporal and spatial patterns during differentiation. Transcriptional activator proteins direct the specificty of the core transcription machinery, but the mechanisms activator proteins use to marshal the core machinery is not understood. One complex that is an integral part of the core transcription machinery is TFIID. TFIID is composed of the TATA-binding protein (TBP) and TBP-associated factors (TAFIIS). Biochemical characterization of TAFIIS indicates several possible functions. However, the in vivo function during transcriptional activation is not clear. The cannonball (can) gene of Drosophila melanogaster encodes a tissue specific TAFII and regulates a program of gene expression essential for spermatid morphogenesis. can is a homolog of dTAFII80 from Drosophila and can expression appears to be restricted to the male germline. The phenotype of can mutants indicates only male germline development requires wild type function. The identification of mutations in a tissue specific TAFII will allow examination of the mechanisms by which TAFIIS function. Target genes regulated by can have been identified and gene expression can be monitored in vivo for the endogenous genes and reporter constructs with altered promoter sequences. In addition, four other genes function together or in a pathway with can. Understanding the mechanism by which can regulates transcription of spermatid differentiation genes will help elucidate the general mechanisms of TAFII function.